1. Field of the Invention
The present invention relates to a successive approximation register A/D converter.
2. Related Art
There has been known a successive approximation register A/D converter in which reference voltages generated by using a plurality of resistors connected in series are compared with an analog input voltage by a comparator to perform A/D conversion. By adjusting the number of resistors to be connected in series, it is possible to perform A/D conversion having a desired resolution. There has also been proposed an example in which the reference voltages are generated by using a current segment. The successive approximation register A/D converter has an advantage that an A/D converter having an optional resolution can be configured with a comparatively-simple circuit.
However, because a conventional successive approximation register A/D converter requires many resistors and switches in its inside, there is a problem that the whole circuit volume of the A/D converter increases. Moreover, in the case of the successive approximation register A/D converter, because a capacitor is connected to the input terminal of a comparator, the input voltage of the comparator may become out of the range of a power-supply voltage. In this case, because the comparator cannot correctly perform comparison operation, thereby exerting a harmful influence against accuracy of A/D conversion.
There has also been proposed an A/D converter improved so as to decrease the number of resistors to be connected in series (refer to Patent Document 1). The A/D converter disclosed in this official gazette divides a D/A converter having a plurality of resistors to be connected in series into two converters. An output of an upper side D/A converter is input to a comparator through a capacitor and an output of a lower side D/A converter is directly input to the comparator. An output signal of the upper side D/A converter is attenuated and input to the comparator due to parasitic capacitance of an input terminal of the comparator. However, an output signal of the lower side D/A converter is input to the comparator without being attenuated. Therefore, there is a problem that the accuracy of the upper side D/A converter is different from that of the lower side D/A converter.